The very oldest men are still interested in sex but illness and a lack of opportunity may be holding them back, Australian researchers reported on Monday.

The "male" hormone testosterone was clearly linked with how often a man over 75 had sex and doctors need to do more studies to see if hormone replacement therapy might benefit older men, the researchers said.

Zoe Hyde of the University of Western Australia and colleagues surveyed more than 2,700 men aged 75 to 95 for their study, published in the Annals of Internal Medicine. They asked a range of questions about health, relationships and sexual activity.

"The older men were, the less likely they were to be sexually active, but sex remained at least somewhat important to one fifth of men aged 90 to 95 years, refuting the stereotype of the asexual older person," they wrote.

"Of those who were sexually active, more than 40 percent were dissatisfied with the frequency of sexual activity, preferring sex more frequently."

More than 30 percent of the men reported some sort of sexual activity in the past year but more than 48 percent said sex was important, suggesting many wanted to have sex but could not.

Age was a factor but so were testosterone levels, the lack of an interested partner and various diseases from diabetes to prostate cancer.

More than 40 percent of the men who had not had sex recently said they were not interested.